The present invention relates generally to over-voltage protection systems and more particularly to over-voltage protection against periodic disturbances.
Communications systems contain vast lengths of wiring that may be subjected to various disturbances. Lightning strikes, power lines, and other interferences may impress high voltages and high currents through these lines. Some of these interferences impart energy sufficient to damage or destroy communications equipment connected to the lines. To protect communications equipment, lines may include current limiting devices, such as fuses, to protect against sustained high voltages and voltage limiting devices to protect against high voltage surges. For example, in the event of a lightning strike, a shorting device may short circuit between the wires of a twisted pair line to allow the energy of the surge to dissipate along the full length of both wires. For a periodic disturbance, such as an impressed alternating current, a shorting device may alternate between short circuiting the wires in either direction. While this may dissipate a significant portion of the energy from the disturbance, the cyclical response of the shorting device to the periodic disturbance can result in high frequency surge remnants that can damage communications equipment. Moreover, the cyclical response may cause the shorting device to heat, causing potential fire hazards and damaging protection circuitry.
In accordance with the present invention, techniques for over-voltage protection against periodic disturbances are provided which substantially eliminate or reduce disadvantages and problems associated with previous techniques. In a particular embodiment, the present invention satisfies a need for a device that protects against a periodic voltage disturbance without passing on damaging surge remnants.
According to one embodiment of the present invention, an over-voltage protection module includes a selectable crowbar device (SCD) that, when activated, permits current to flow from a first terminal to a second terminal. The protection module also includes a control module having a first state and a second state. In the first state, the control module activates the SCD and enters the second state if a voltage differential between the first terminal and the second terminal exceeds a first activation threshold. In the second state, the control module activates the SCD if the voltage differential exceeds a second activation voltage threshold that is less than the first activation voltage threshold.
In accordance with another embodiment of the present invention, a method for protecting from over-voltage provides a first state having a first activation voltage threshold and provides a second state having a second activation voltage threshold that is less than the first activation voltage threshold. The method receives a signal that produces a voltage differential between a first terminal and a second terminal. In the first state, the method enters the second state and activates a selectable crowbar device if the voltage differential exceeds the first activation voltage threshold. In the second state, the method activates the SCD if the voltage differential exceeds the second activation voltage threshold, wherein activating the SCD permits current to flow from the first terminal to the second terminal.
Various embodiments of the present invention provide numerous technical advantages. A device implementing these techniques provides protection against periodic disturbances without creating high frequency power fault harmonics. This reduces the possibility that cyclical disturbances will result in damage to communications equipment. Also, a reduced response time to subsequent cycles of a periodic disturbance reduces stress on an over-voltage protection module. The module subjects itself to less energy, and thus dissipates less heat than traditional protection circuitry when experiencing a periodic disturbance. This guards against damage to the protection module, increases expected life spans, and reduces fire hazards. Moreover, while the increased complexity of the device can increase material and production costs, these costs may be more than offset by the value of equipment protected from damage.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.